The warning came in a four-page memo to the Deputy Secretaries of Defense and Transportation from retired Admiral Thad Allen, Chair of the advisory board. The memo was nominally a report of the board’s April 2024 meeting in Colorado Springs.

The overwhelming majority of Allen’s message, though, dealt with GPS and U.S. PNT being vulnerable, the importance of PNT to the nation’s safety and security and the failure of the government to do the things it said it should and would do. It says:

“America’s continued over-reliance on GPS for PNT makes critical infrastructure and applications vulnerable to a variety of well-documented accidental, natural and malicious threats.

…our conclusion is that PNT, in general, and GPS, in particular, have not been accorded their rightful prominence in the national policy agenda.

Simply put, the Board believes that the 20-year-old framework for GPS governance and the current policy statements establish neither the priority that the system deserves nor sufficiently clear accountability for its performance.”

The reason for this was assessed to be that the leadership and governance structure established by 2004’s NSPD-39 and confirmed in 2021’s SPD-7 was not working.

Allen gave a recent policy document on critical infrastructure as an example. All critical infrastructure sectors use PNT, and most depend on it.

“These findings were reinforced just earlier this year by the release of the National Security Memorandum on Critical Infrastructure Security and Resilience (NSM-22, April 30, 2024). We were surprised to discover that GPS is nowhere mentioned in that important document.”

While not mentioned in the memo, PNT was also not mentioned in national cybersecurity documents issued last year. This is despite timing being essential to the operation of IT systems, and time and location data being key elements in many applications.

Allen’s memo also emphasized that the real issue is not about GPS, but overall national PNT capability. He went so far as to say, “Space Policy Directive 7 (SPD-7), U.S. Space-based PNT Policy, must be revisited and the term ‘space-based’ should be removed from its title” (emphasis added).

Reinforcing this message, the following table comparing U.S. and Chinese national PNT systems was included in the memo.

“The Board believes it is time to take a fresh look at our approach to PNT governance and establish a clear strategy for re-establishing an unquestioned position of leadership for the United States. To be successful, such a strategy requires a governance structure characterized by clearer authority and accountability. Ideally, the administration should propose legislation to Congress that would support this goal with a clear mandate (authorization) and resources (appropriations) adequate to the task.”

As part of his report of the April meeting, Allen’s memo reaffirmed the board’s nine recommendations from January of last year and added three more:

• Direct the U.S. Space Force to establish a way for “good enough” monitoring of GPS L5 signals using existing capabilities and set L5 signals healthy subject to a “use at your own risk” caveat, just as is done with L2C today. We recommend establishing a deadline of September 2024.
• Direct federal Chief Information Officers to acquire and install multifrequency dual-system (GPS-Galileo) receivers to complement GPS use. The Department of Transportation and the Department of Homeland Security should actively encourage critical infrastructure users to adopt multifrequency dual-system (GPS-Galileo) receivers.
• Assign a lead agency and single individual with clear responsibility and authority for the end-to-end prompt detection, characterization, and removal of significant sources of interference to GNSS in the United States.

Allen’s memo comes on the heels of a similar warning to United States leaders about the nation’s vulnerability to GPS disruptions. On July 18, the

National Security Space Association published a report titled “America’s Asymmetric Vulnerability to Navigation Warfare: Leadership and Strategic Direction Needed to Mitigate Significant Threats,” which cited several national PNT shortfalls from the Federal Radionavigation Plan as significant concerns.

Two of that paper’s key findings are:

“Long-standing lack of progress on issues important to U.S. national, homeland, and economic security,” and

“Focused leadership, properly empowered and resourced, is essential to the national PNT strategy’s success.”

It warned that significant disruption to GPS “…could have cascading effects which unravel America’s socioeconomic fabric…”

<p>The post Experts to PNT leaders: “It’s not working!” first appeared on GPS World.</p>

These are just three of the efforts the U.S. government is pursuing as a result of recommendations from the President’s National Space-based Positioning, Navigation and Timing (PNT) Advisory Board.

For 20 years the PNT Advisory Board has been providing the government independent expert advice about GPS and PNT.

Established by presidential directive in 2004 and administered under the Federal Advisory Committee Act by NASA, its charter has been regularly renewed. The charter provides that the board shall:

• Be composed of experts from outside the United States government.
• Seek input from state and local governments, industry and academia on developments in the application of space-based PNT technologies.
• Evaluate national and international needs for changes in space-based PNT capabilities and assess possible trade-offs among options.
• Provide independent advice and recommendations to the National PNT Executive Committee (co-chaired by the Deputy Secretaries of Defense and Transportation) on policy, system requirements, and program needs.

While “space-based” is in its name and charter, the board has long recognized that terrestrial assets also can play an important role in serving PNT users by augmenting, reinforcing, and complementing GPS. The use of complementary systems, for example, could help demotivate intentional jammers and spoofers and help safeguard users during any interference event. Thus, the board often considers a wide range of capabilities and systems.

The board also discusses policy, education, international relations and other issues important to the PNT community. As one board member commented, “Technology doesn’t exist in a vacuum. It is developed by, and intended to serve, people. If you don’t recognize that, you are missing most of the picture.”

The current board’s membership includes an impressive array of experts in PNT policy and technology. Its 29 members include a former governor, a retired admiral, three retired generals, GPS’ original chief architect, a former undersecretary, a former assistant secretary, three former presidents of the Institute of Navigation (ION), three international members and experts from across academia and industry.

Chaired by former Coast Guard Commandant Admiral Thad Allen, the board’s primary efforts are driven by its six subcommittees, reflecting a holistic approach to effective PNT:

• Strategy, Policy & Governance
• Protect, Toughen & Augment
• Emerging Capabilities, Applications & Sectors
• Education & Science Innovation
• International Engagement
• Communications & External Relations

While the subcommittees meet in fact-finding sessions to gather data, the PNT Advisory Board’s deliberations are public. Semi-annual meetings in  Washington, D.C. and other locations may be attended by anyone, either in person or virtually. Announcements on the board’s webpage and in the Federal Register provide details before each meeting. By law, the minutes of each meeting are available to the public, and video recordings of meetings are normally posted as well.

Input from the public about PNT issues of concern is also welcome to inform the board’s current and future deliberations. Information on how to send input will be posted with the meeting announcement here. 

According to board member Jeff Shane, former undersecretary at the U.S. Department of Transportation (DOT), the PNT Advisory Board is evidence of government at its best. “The very fact that the board was established underscores our government’s willingness to hear and consider the widest variety of views and input. It should be a source of optimism, and even pride, for the entire PNT community.”

National Space-based PNT Advisory Board

The next meeting will be from 9:00 a.m. to 6:00 p.m. MDT, April 24, 2024, and from 9:00 a.m. to noon April 25 at The Antlers Hotel in Colorado Springs, Colorado. Click here for information on a reception on April 23, featuring Gen. David Thompson.

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Brad Parkinson

We, of the PNT universe, have been hearing a rather continual message of doom from the media regarding the fragility of the GPS (or GNSS) signals. In a way, they are right. The received GPS signal is 1/10th of 1 millionth of 1 billionth of a Watt. It can be susceptible to jamming and spoofing.

In response, the U.S. government has sponsored major studies and some competitive tests of techniques to augment or possibly replace GPS. I applaud such queries, but also would strongly advocate more balance in efforts to increase robustness of positioning, navigation and timing (PNT).

Specifically, I argue for increased emphasis on well-known techniques that can greatly toughen GNSS receivers to both jamming and spoofing. Some of these techniques are deliberately denied to civil users by government policy.

Background

The PNT Advisory Board (PNTAB) is a panel of national experts who report to the PNT ExCom. The ExCom is comprised of the deputy heads of the nine U.S. government departments with the largest stakes in PNT. The PNTAB has a starkly simple and well-stated goal:

To meet its overarching goal, the PNTAB has developed a three-legged strategic framework, known as “PTA”: “We must protect, toughen and augment GPS to ensure that it continues to provide economic and societal benefits to the nation.”

Most current U.S. government efforts have been focused on the third of the PNTAB strategic legs: augmenting the GPS system. These system augmentations include: modernized Loran (eLoran), fiber-optic distribution of time, and ranging to low-Earth-orbit (LEO) satellites (particularly the swarms of communications satellites). In general, these system augmentations offer no hope of being equivalent to GPS in terms of availability and accuracy.

However, augmentations have the advantage of either being less vulnerable to interference, or highly proliferated in case of satellite outages. As supplements, or in an emergency, they can perform a very valuable role, but with nowhere near the equivalent performance of normally operating GNSS, which can routinely provide worldwide, 24/7 precisions better than decimeters in the dynamic real-time kinematic (RTK) mode. In the United States, GPS also offers continuous, real-time integrity assessments courtesy of the FAA¹. Europe has a similar, compatible integrity system called EGNOS, and there are other regional system augmentations.

In summary, the current PNTAB assessment regarding these substitutes is:

“No current or foreseeable alternative to GNSS (primarily GPS) can deliver the equivalent accuracy (static down to millimeters) and worldwide, 24/7 availability.”

Toughening User Equipment

Toughening, the second leg of our assurance strategy, includes all aspects of GPS enterprise vulnerability — satellites, ground control and user equipment. For this article, I am focused on toughening the user equipment. I would argue that we have largely under-emphasized, or been prohibited by national policy from using, well-known and widely available user equipment toughening technology.

The main vulnerabilities of GPS receivers are jamming and spoofing of the received signals. Familiar anti-jam (A/J) methods can substantially overcome the inherent weakness of GPS signals to defeat deliberate jamming and spoofing. As I outline here, such measures can reduce a jammer’s effective radius by a factor of more than 100 and reduce the effective jammer area by a factor of 10,000 compared to the unprotected receiver.²

Thus, these methods are also deterrents, because they can render ineffective such hostile (or possibly inadvertent) acts. Further, the technology that provides this significant toughening is available now or will be within a few years, rather than the many years required by some alternative, system-level augmentations.

Toughening techniques (A/J improvements) are traditionally calibrated as the improvement in the amount of jamming that can be tolerated, measured by the jamming-to-signal power ratio (J/S) expressed as decibels (dB). However, for this discussion, I will also use a different, more intuitive, measure. This metric is the Denial Radius Reduction Ratio (DRRR):

DRRR = (radius of jammer denial after J/S measure applied)/(jammer radius without improvements)

For example, a 15-dB improvement in J/S would lead to a DRRR of 0.178.³ In other words, the 15-dB improvement has reduced the denial radius to about 18% of the line-of-sight radius that would be denied to an untoughened receiver. Note that the simultaneous use of techniques is generally multiplicative. For example, simultaneously applying technique #1 with a DRRR₁ of 0.5 and technique #2 with a DRRR₂ of 0.3, would result in a DRRR_1&2 of 0.3 *0.5, or an overall DRRR of 0.15. This is the advantage of using this metric to describe the A/J improvements. ⁴

Baseline Case

For our basis for comparison, we will consider the L1 C/A signal in full accuracy (State 5) tracking mode and a 1-kW noise jammer.^5,6 For this situation, the line-of-sight jammer could deny GPS to a radius of about 560 kilometers. A discussion of the lower accuracy State 3 tracking is included below.
It is useful to consider toughening techniques in four major categories.

Toughening Category 1: Signal Processing. With L1 C/A, GPS receivers can improve jamming resistance, albeit with loss of ranging (tracking) accuracy, by using code tracking mode – State 3. This reduces a line-of-sight jammer’s denial radius (DRRR) to about 0.29 (a 10.7 dB improvement).

Toughening Category 2: Inertial Components and Very Stable User Clocks. This includes miniature micro-electromechanical (MEMS) components up to high-grade inertial measurement units (IMUs) and quartz to chip-scale atomic clocks (CSACs). These techniques enable narrower tracking filters and longer averaging, as well as allowing navigation through regions when GPS is denied. The range of DRRRs is 0.40 down to 0.10. We will use a nominal value of 0.18 (a 15-dB improvement).

Toughening Category 3: CRPAs. Controlled reception pattern antennas (CRPAS) are digital, multi-element, phase-steered antennas. They represent well-understood and available technology; they have been used in large surface-search radar systems for many decades.⁷ They can be used in null-steering or beam-steering modes.⁸ The number of antenna elements could range up to dozens. Potentially, they could produce DRRRs down to .01 — that is, a 99% reduction of jammer radius to 1% of the unprotected GPS receiver value.

Unfortunately, the U.S. government does not allow more than three-element CRPAs to be manufactured or sold for civil use. ⁹ This is due to some very old International Traffic in Arms Regulations (ITAR). For our nominal example, we will assume the restriction has been relaxed and use a CRPA of about 20 elements, which should produce a DRRR of 0.06 (a 25-dB improvement).

Toughening Category 4: Signal Alternatives. This category includes alternative modulations at 1575 MHz (L1C, Galileo or other GNSS) and alternative frequencies (L5, L2, Galileo). Note that the modern signals generally offer significantly improved signal-processing toughening as well as increased power.

Using L1C in State 3 compared to L1 C/A in State 5 would yield a DRRR of 0.10. (a 20.3-dB improvement). The L1C international signal should be operational on GPS by mid-decade. The L5 signal, at 1176 MHz, is clearly the most capable of the civil GPS signals in terms of jam resistance. L5 also should be declared operational by mid-decade. As the use of LEO communication satellites matures, their use may also fit this category.

Summary of Receiver Toughening Options. Quantification of the selected, nominal receiver augmentations are summarized in FIGURE 1 for both full accuracy (State 5, centimeter-level accuracy in RTK) and for less accurate code tracking (State 3, meter-level accuracy). These results are shown with a logarithmic scale to accommodate the wide range of denial radii.

Figure 1. Effect of receiver augmentations on accuracy for both State 5 and State 3. (Image: Brad Parkinson)

The example shows that a 1-kW hostile jammer’s denial radius¹⁰ can be reduced by a factor of about 100, using the conservative example augmentations of inertial and CRPAs. Because area is proportional to radius squared, the effective denial area of an augmented receiver would be 1/10,000th of the unaugmented receiver, using the example values.

Reverting to code-only (State 3) tracking, it enables operating through higher levels of jamming, albeit with less ranging precision. All these receiver augmentations and tracking techniques would also offer a significant defense against any attempt to spoof (deceive) the position measurement. Again, none of these techniques are new; we demonstrated the capabilities at the original GPS Joint Program Office in 1978, more than 40 years ago. Today, many competent manufacturers are offering toughened GPS receivers with combinations and variations of these techniques.

GPS jamming tests at White Sands have caused aircraft interference, which could be largely avoided with toughened receivers. Here, M-code is tested on Joint Light Tactical Vehicle platforms in 2020. (Photo: Joe Bullinger/U.S. Navy)

Meeting Increasing Threats

Threats of both jamming and spoofing seem to have accelerated. Devices to perform these illegal acts are freely advertised on the internet. In fact, we read of incidents both in the United States and abroad.¹¹ Near White Sands Missile Range in New Mexico, there have been GPS air traffic control outages due to authorized military operational jamming exercises. Such interruptions could be largely avoided if more robust (toughened) GPS receivers, with the enhanced jam resistance techniques outlined here, were in use.

News reports also highlight the spoofing issue. Hardening against this threat is also a task for toughening. A serious spoofing sequence usually starts with a strong jamming signal to cause the user’s receiver to break lock, followed by a strong false GNSS signal that causes false lock by the receiver. Using the false signal leads to a false position, of course. The first line of defense is to avoid the break-lock threat. Failing this, numerous self-check and authentication schemes can be used to avoid false positions.

A conclusion is that avoiding the break-lock jamming is a first line of defense against a spoofing attack. Of course, the toughening techniques to avoid this are the main subject of this paper. One well-known expert has stated that, for a well-designed receiver, a spoofing attack might deny the measurement of position, but should never cause false PNT. I will leave further discussion of spoofing to other authors.
Returning to disruptions of service in general, some have suggested many interference occurrences have gone unreported, because the typical user would not know where to make such a report. To remind the reader, the official reporting center is online at www.navcen.uscg.gov/?pageName=gpsUserInput.

In addition, the U.S. Federal Communications Commission (FCC) has repurposed a portion of the spectrum adjacent to the main GNSS L1 frequency (1575 MHz). The agency is converting the license holder’s original authorization to transmit a weak space-transmitted signal into a much stronger terrestrial system, potentially with thousands of transmitters. Extensive testing of civil GPS receivers by the U.S. Department of Transportation demonstrated that the planned repurposing will interfere with many existing receivers. Some observers call this disruption “legal jamming.”

Such a new spectrum use could have grave impacts on those existing receivers, notably aviation (especially helicopters and UAVs) and first providers. On the other hand, installing toughened replacement receivers would make the users virtually immune to this threat.

So, this begs the question: If the receiver toughening techniques are so effective, why are they not more prevalent?

Barriers to Adoption

Let’s examine the potential resistance to more extensive use of receiver augmentations.

Knowledge. This involves underestimating the threat to PNT and not understanding that toughening techniques are available. As mentioned above, threats to the fragile GNSS signals are growing.

There seems to be little interest in the U.S. government to monitor and suppress interference in the United States. Internationally, the reported incidents continue to increase.¹² It is also reported that certain European aircraft manufacturers have installed advanced, deeply integrated inertial systems with civil GNSS receivers to defeat or “flywheel” through radio-frequency threats (particularly in the Middle East).

As this threat trend continues, GPS manufacturers and users must realize that many of these solutions will take time to authorize, implement and install. It appears that the media are not aware that not only are the toughening techniques outlined here feasible, but many manufacturers have product offerings that address these threats. Having off-the-shelf solutions will give the PNT user the opportunity to retrofit and defeat such threats.

Cost. The cost for a receiver to revert from State 5 to State 3 is zero, and all receivers that use Code 5 (for example, RTK) would naturally have this built in. Regarding use of other frequencies (such as L5) and modulations (L1C) rather than the original L1 C/A, there is some small cost associated, including the additional antenna for L5. Note that all modern cellphone chips, such as Qualcomm’s, have this capability — including integrated carrier-phase measurements — in a chip that is estimated to cost about $5. A potential barrier is that the L5 and L1C signals are not yet declared operational, but these newer GPS signals should be operational within about five years.

The costs of many inertial components (accelerometers and gyros) have plummeted in the last few decades with the proliferation of MEMS devices, particularly into cellphones and automobiles. Their power consumption has also decreased while their performance has steadily improved. Full IMUs are much more expensive, but are already installed on many commercial aircraft. Robust toughening with inertial sensors can be achieved, but requires deep integration and careful engineering.

Depending on their complexity, CRPA antennas can be a costly receiver augmentation. Very high-speed (330 MHz is available), 16-bit, A-to-D converters are at the heart of most of these phased-array devices. Some are priced at about $150 each. Applications with a high premium for PNT availability in the face of interference — such as commercial aircraft and cargo ships — should find them affordable. Aircraft manufacturers have resisted retrofitting existing aircraft with larger diameter CRPA antennas because of costs. For some of these applications, integration costs can be more than the costs of the receiver itself, particularly if not included in the original manufacture.

As the yearly sales of fully toughened receivers increase, the economies of scale should significantly reduce unit costs. Each application will make its own determination of affordability, based on risk.

Government restrictions. Civil use of CRPAs with four or more elements is restricted by ITAR. These are well-meaning restrictions on technologies that could be used against the United States by hostile military forces. Unfortunately, the phased-array antenna techniques are not only well understood and tested, but relatively inexpensive components are widely available on the open world market. In particular, the restriction on the number of CRPA elements for civil use should be completely removed. All potential enemies are well aware of the beam-steering method and have ready access to the parts to build them. Thus, the restriction is only harming civil users without affording any apparent improvement in general military posture.

Certified aviation receivers need approval for deep integration of inertial systems and multi-element CRPAs. (Photo: JasonDoiy/iStock/Getty Images Plus/Getty Images)

Gaining permission: FAA flight certifications. To be used in commercial aircraft operations, navigation equipment must be certified by the U.S. Federal Aviation Administration (FAA). Current, certified GPS aviation receivers have rudimentary toughening techniques, but gaining approval for deep integration of inertial systems and multi-element CRPAs must be completed. It is gratifying to hear that work is underway to do this.

Any civil solution for the United States must expand integrity monitoring beyond GPS to include all GNSS, and must be operationally included in the FAA’s integrity monitoring with WAAS.

Recommendation

In describing resistance to interference, I have introduced the idea of DRRR – Denial Radius Reduction Ratio. Also, I have used a 1-kW white-noise jammer as a standard threat for calculating the denial radius of various GPS receiver configurations. My recommendation is that equipment manufacturers specify their receiver offerings by stating their equipment’s denial radius against a “standardized” 1-kW EIRP white-noise jammer.

Summary

Media reports of interference to GPS may be accurate, but they generally do not recognize that available toughening techniques can largely defeat those interference threats. While exploring systems-level replacements or augmentations (such as LEO ranging or Loran) is worthwhile, GPS (or GNSS) still offers the greatest capability in combined terms of accuracy, integrity and coverage.

The goal of all PNT providers — GPS operators, certifiers and manufacturers — should be assured PNT, with the expected accuracy and availability. The described toughening techniques to do that have been known for decades, but have not been generally adopted by many critical civil users. Many manufacturers do offer civil products under existing government constraints.

The purpose of this article was to describe and advocate the solutions available to increase the robustness and toughening of civil GPS receivers. For example, readily available toughening augmentations for civil receivers can reduce the denial radius of interference by 99% or more. This implies that any denied area would be squeezed down to 1/10,000th of that experienced by an unaugmented receiver.

The payoff is high, and should be affordable to many high-end, safety-of-life users. Therefore, a renewed focus on toughening of GPS receivers is overdue. We discussed barriers to rapid adoption but, more than the specifics, it is crucial to fully and urgently embrace the goal of toughening receivers, particularly removing the ITAR restriction on antennas.

Opinions expressed in this article are those of the writer and should not be construed as the official position of the PNTAB or any U.S. government organization.

Notes

1. While not a part of the U.S. Department of Defense’s GPS operation, the FAA’s integrity signal (WAAS) is a GPS-type signal directly available and being used by almost all modern GPS receivers, including cellphones.

2. This is the ration of Denial Radius between, for example, unaugmented L1 C/A in State 5 and augmented L1C in State 3. Please see later footnote and graph.

3. Calculated as 10 ^(–15/20)

4. Of course, the more traditional dB measure of jammer resistance can (in most cases) be simply summed to estimate the total effectiveness. The use of DRRR gives a more intuitive calibration, particularly for non-technical persons who may not be at all familiar with dBs.

5. State 5 is the tracking mode that provides full accuracy; it requires tracking both the PRN code and the reconstructed carrier. It is required for RTK positioning, which is usually used for automatic control of machines or vehicles. It is most vulnerable to interference. Less vulnerable is State 3 tracking, which only provides code tracking, with precisions of perhaps a few meters.

6. Deliberate jamming using “matched spectrum” GPS-like modulations have also been employed in the Middle East. The toughening techniques described are also generally applicable, with appropriate rescaling. The matched spectrum is fundamentally used to improve the jammer spectrum efficiency.

7. See Michael Jones, “Anti-jam systems: Which one works for you?”, posted on gpsworld.com on June 14, 2017, for a survey of manufacturer offerings at that time. Named companies generally continue to offer improved, jam-resistant products. 

8. Phased-array antennas, by their nature, distort phase and would probably have to be calibrated for precise operations such as RTK. Fortunately, we understand that this problem has been reportedly addressed and largely solved by the U.S. Navy’s JPALs program.

9. See U.S. Munitions List at www.ecfr.gov/current/title-22/chapter-I/subchapter-M/part-121.

10. The denial radius results shown can be easily scaled for weaker or more powerful jammers. The scaling goes as simply the square root of the power ratio of a different size jammer to the 1-kW example. A 10-watt jammer is 1/100th the power of the example. The denial radius would then be one tenth of the example, which is the square root of 1/100.

11. “Ships have reported an increasing number of cases of significant GPS interference and jamming in recent months. The geographic areas with more than one reported incident include the eastern and central Mediterranean Sea, the Persian Gulf, and multiple Chinese ports.” (Source: www.gard.no/web/updates/content/30454065/gps-interference-and-jamming-on-the-increase).

“North Korea is using radio waves to jam GPS navigation systems near the border regions, South Korean officials said. The broadcasts have reportedly affected 110 planes and ships and can cause mobile phones to malfunction.” (Source: www.bbc.com/news/world-asia-35940542).

12. “Reports of GPS outages submitted by pilots from the cockpits of commercial flights show that disruptions to the navigation system, which was created and is maintained by the U.S. government, are now standard occurrence on the flight routes between North America and Europe and the Middle East, according to data from the European Organization for the Safety of Air Navigation, known as Eurocontrol.” Fortune Magazine, Nov. 1, 2020.

<p>The post Toughen GPS to resist jamming and spoofing first appeared on GPS World.</p>

John Fischer

“The U.S. government already has the National Executive Committee for Space-Based PNT (see gps.gov), which coordinates policy among all the branches of government. There is also a PNT Advisory Board, which includes some international members to inform the committee, and a National Coordination Office to execute the policies. This is in addition to the new U.S. military’s Space Command under which GPS operates. So, space-based issues are being addressed, but maybe more could be done to coordinate terrestrial-based PNT systems with space-based ones.”
— John Fischer

Ellen Hall

“As new technology advances, government, along with its policies and laws, struggles to keep pace. This was certainly true of the internet and cybercrime. This is also true of terrestrial-based PNT, where new technology emerges in areas not currently covered. Policy gaps and overlapping technologies need to be addressed and coordinated. It certainly seems that this would be one of those areas that could benefit from oversight. Space-based PNT is currently addressed by the National Executive Committee for Space-Based PNT, among others.”
— Ellen Hall

Bernard Gruber

“The threat to U.S. space-borne assets and the signals they generate is very real. Both commercial and military users of GPS continue to seek independent alternatives to current PNT systems that are diverse and robust. The National Coordination Office supports the National Executive Committee (NEC) for Space-Based PNT. I believe the role of the NEC should be broadened to include terrestrial and alternative PNT as a first step to charter/secure a new or expanded program office.”
— Bernard Gruber

Editorial Advisory Board

Tony Agresta, Nearmap 

Miguel Amor, Hexagon Positioning Intelligence

Thibault Bonnevie, SBG Systems

Alison Brown, NAVSYS Corporation 

Ismael Colomina, GeoNumerics

Clem Driscoll, C.J. Driscoll & Associates

John Fischer, Orolia

Bernard Gruber, Northrop Grumman

Ellen Hall, Spirent Federal Systems

Jules McNeff, Overlook Systems Technologies

Terry Moore, University of Nottingham

Mitch Narins, Consultant

Bradford W. Parkinson, Stanford Center for Position, Navigation and Time

Stuart Riley, Trimble

Jean-Marie Sleewaegen, Septentrio

Michael Swiek, GPS Alliance

Julian Thomas, Racelogic Ltd.

Greg Turetzky, Consultant

<p>The post Editorial Advisory Board Q&A: Do we need a new PNT office? first appeared on GPS World.</p>

Dana Goward, President, Resilient PNT Foundation

Letter to the Editor

February 2022

In November’s issue of GPS World, Editor-in-Chief Matteo Luccio opined that eLoran is part of the solution to GNSS vulnerability.

In January’s issue, he listed 10 questions from a PNT expert perhaps unfamiliar with eLoran.

These are important questions that must be asked of any technology, especially one under consideration to augment and back up our essential, but very weak and vulnerable, GNSS signals.

Yet the expert’s concerns pale in comparison to the essential questions about GNSS and PNT facing the United States and the West.

While I look forward to answers to the “10 questions” as a part of our ongoing professional dialogue, there are two important points of context we all need to keep in mind.

A Broad Consensus

First, Mr. Luccio’s assertion about eLoran being a part of the solution is more than reasonable. It also has a lot of impressive support from a wide variety of authoritative sources.

In 2008 and 2015, after much study each time, the U.S. government decided on and committed to building eLoran systems. Also, the U.S. government-sponsored National Space-based Positioning, Navigation and Timing (PNT) Advisory Board recommended eLoran in 2010 and 2018 as a part of securing the nation’s critical PNT capability.

In 2021, the U.S. Department of Transportation told Congress that wide-area terrestrial broadcast was a necessary part of a national PNT architecture. They later commented that infrastructure required per coverage area would be a key selection criterion for that broadcast technology. In other words, a system like eLoran.

Overseas, support for Mr. Luccio’s statement on eLoran is even stronger.

• The United Kingdom has long endorsed eLoran and operates an eLoran transmitter as a timing reference.
• Russia operates Chayka, a version of Loran.
• Available information points to Iran’s terrestrial PNT system being a form of Loran or eLoran.
• China and South Korea have long had Loran-C systems, and both are in the process of upgrading to the eLoran standard.

Each of these countries has publicly announced that it operates Loran/eLoran as a matter of national security in case space-based systems are jammed or destroyed, and to generally avoid overdependence on space-based PNT signals.

So, Mr. Luccio’s assertion was not at all revolutionary. Given all the studies, recommendations and existing uses, it would be surprising if he did not consider eLoran a part of the solution.

The Important Questions

Second, modern keying, encryption, authentication and other tech advances will help make all PNT technologies much safer and more resilient than they would have been decades ago, Loran and eLoran included.

Yet all will still have their strengths and weaknesses.

The most important questions we must ask are about how to establish the right level of national PNT security. These include:

• What is the right combination of technologies and systems with different delivery and failure modes that complement and reinforce GNSS and each other?
• How can the systems be efficiently and effectively implemented?
• How can the services they provide be easily accessed and widely adopted to ensure all parts of society are protected?

Countries such as China have answered these questions and are well down the path to implementation and wide adoption. Their robust national PNT architectures support easier rollout of 5G, rural broadband and other systems. They also serve as solid tech infrastructure upon which to build myriads of technologies and applications yet to be conceived.

Those nations not so advanced must accelerate their efforts. Otherwise, they must resign themselves to perpetually coping with GNSS vulnerabilities, including the possibility of attacks, and an eventual second or third place in the world because of their shortsightedness.

Dana A. Goward, President
Resilient Navigation and Timing Foundation

<p>The post Worry about PNT and national security, not just eLoran first appeared on GPS World.</p>

Speaking to a gathering of space industry leaders, Dr. Brad Parkinson, the original chief architect for establishment of the Global Positioning System, outlined the threats to GPS signals posed by a proposal from Ligado Networks that is before the Federal Communications Commission (FCC).

The proposal, which was initially approved, has been pending since 2011 over concerns about its impact on reception of GPS signals.

When asked why the proposal has been pending for so long, Parkinson speculated a combination of the enormous sums of money potentially involved and difficulties for the FCC in arriving at an equitable solution were to blame.

“I hope they find some spectrum for Ligado,” he said. “Just not the spectrum they are asking for.” Referring to the powerful proposed transmissions in frequencies adjacent to those used by weak GPS signals, he said “You don’t want to put a rock band next to a retirement home.”

“You don’t want to put a rock band next to a retirement home.”

Parkinson was speaking to the National Space Council’s User Advisory Group (UAG). The UAG was chartered under the Federal Advisory Committee Act in December 2017. Membership includes the CEOs of Lockheed, Boeing and other companies with interests in space, the Governor of Alabama, five former astronauts, and other leaders in space technology and policy.

This was the first time the UAG’s agenda focused on GPS and positioning, navigation, and timing issues. Parkinson was invited to discuss the activities of the U.S. National Space-based Positioning, Navigation, and Timing Advisory Board. The UAG was interested in learning from the PNT Advisory Board’s experience, and about some of the issues it has encountered over the 15 years since its establishment in 2004.

Parkinson used the PNT board’s experience with adjacent band compatibility and the Ligado proposal as an example of a particularly thorny issue. He said the Ligado issue was an ongoing concern that the UAG should share.

Brad Parkinson shared his concerns over lack of Ligado decision with the National Space Council’s User Advisory Group (UAG). (Photo: Rebecca Zia)

Also that spectrum issues writ large should be of great concern to all users of space as the competition for spectrum from terrestrial users will continue increase. He called the Ligado proposal “a grave threat to GPS,” especially for aviation and high-precision users.

During the course of the presentation Parkinson asked for the UAG’s support of the PNT Board’s recommendations on the Ligado proposal. He also asked the group to endorse the National PNT Executive Committee’s adjacent band compatibility methodology to any future such proposals.

Parkinson’s presentation concluded with a discussion of how GPS is increasingly being used to support activities in space. It has long been used for booster guidance during the initial phase of space flight.

Relatively recent work using side lobe signals from GPS satellites has caused engineers to recognize a Space Service Volume for GPS. This allows use of the signals for mid-course launch corrections, satellite station-keeping, rendezvous, and potentially trans-lunar navigation.

Parkinson’s presentation is available online.

Feature photo: Rebecca Zia

<p>The post Parkinson questions long delay resolving Ligado issue first appeared on GPS World.</p>

The U.S. Air Force will load a new signal feature, designed to make spoofing detectable, aboard a satellite that will broadcast it from space as a security overlay for the GPS L1C signal, but not until 2022 at the earliest.

The Chips Message Robust Authentication (Chimera) is now in testing under the auspices of the Air Force Research Laboratory (AFRL), getting ready to fly on the Navigation Technology Satellite 3 (NTS-3), which will trial a number of new PNT techniques and technologies.

Chimera inserts encrypted digital signatures and watermarks within the L1C signal. A GPS receiver with the requisite additional capability for this purpose can then detect whether the signal is real or fake and also authenticate the location of a GPS receiver that is remotely located.

This key feature could provide a defense against hacking by blocking access from anyone unable to prove they are at an anticipated or licensed site. Hacking, of course, is a growing threat to all sorts of infrastructure: financial, security, utility grid and more.

Presentation slide from PNT Advisory Board briefing by Logan Scott.

Consultant Logan Scott first proposed the Chimera technology in 2003, when he affirmed that “Some of the spoofing detection measures in wide use offer a false sense of security. Authenticatable signal architectures are needed.” In June, he made a presentation to the PNT Advisory Board: “The Role of Civil Signal Authentication in Trustable Systems.” The two slides accompanying this article appeared in that presentation.

“Chimera represents a fundamental paradigm shift in PVT security paradigms,” Scott related in a subsequent conversation. “Trust takes time and memory on a personal level and, in this case, in GNSS signals, too.

“You don’t trust somebody as soon as you meet them. Over a period of time, you get to know them. If you can’t remember anything, you can’t develop trust either.”

“In the GNSS world, there are a lot of applications where you don’t need output in real time,” Scott said. “For example, to align an inertial. The inertial provides the real-time aspect. You don’t want to send anything to the IMU that is factually incorrect. When building to aid inertial, I can afford to have a delay from real time as long as I tell it where it was 10 seconds ago. The power of that is, if I don’t have to give real-time output, I can ponder and think about things.

“If a spoofer attacks, there’s an evolution that happens there. If I, as the receiver, can see the developing scenario, and how it starts to look at little screwy, I can stop and not send anything to the IMU that might corrupt it.”

How It Works. The core concept of Chimera involves the satellites sending encrypted watermarks, encoded into the signal by the satellite. After a slight delay, the satellite sends the key used to generate those encrypted watermarks. Once a key is sent, the system changes the key.

Since the receiver has already recorded the signal with its watermarks before the key is sent, spoofers cannot know the correct key ahead of time, in time to insert correct watermarks of their own. This means that any spoofed signals can be easily spotted: either the subsequent key won’t match up with the spoofed watermarks, or there will be no watermarks at all.

“Another reason it’s hard for someone to generate these watermarks on their own is because the signal is buried below the noise,” added Scott. “The watermarks are hidden.”

A number of different time delays between signal and key are possible within this concept and within the general set-up of GPS. Scott and the AFRL have, for various practical reasons, provisionally settled on a 6-second delay on the fast watermark channel and a 3-minute delay for the slow watermark channel.

The signal enhancement could be incorporated into the Wide Area Augmentation System (WAAS). This has yet to be fully determined, but this route would lead to a faster implementation of Chimera. Scott thinks that going the WAAS route could bring Chimera capability into action within two years.

The AFRL, however, is looking at a much longer timeline. The NTS-3 satellite, where it first intends to test Chimera, will not launch until 2022 — three years hence. And that’s only a test, not an enactment or a system-wide implementation.

Slide: Logan Scott

Verification. One key benefit for commercial entities, particularly those in financial infrastructure and other systems that increasingly fall victim to hacking, is that Chimera gives them the ability to verify customers’ or partners’ locations before granting any kind of access. The customer’s or other erstwhile user’s GPS receiver would record the full signal, including the watermarks, and transmit that data to the company, entity or data center needing location verification, before the keys are published. Each combination of watermarks and signals is unique to the place where it was recorded, thus it is possible to tell whether the user is actually where they say they are, or in an authorized or pre-identified location before granting access or accepting further input (such as commands).

Scott claims that Chimera affords a 99.9% probability of detecting spoofers. “I have a 99.9% chance of detecting that the watermark is not there, because they don’t know how to generate it. This is based on how you’re processing the signal. It’s designed to be very flexible in how the receiver uses the signal.”
Just One Problem. Receiver manufacturers will have to develop new Chimera-capable receivers, and customers will have to buy them. An additional cost for the added processing, above and beyond that required for normal GPS operation, is unavoidable.

And a Hiccup. Chimera, while an acronym, is as a name perhaps not a totally felicitous choice. In Greek mythology, the chimera is a fire-breathing female monster with a lion’s head, a goat’s body, and a serpent’s tail. These historic ancestors have evolved into the word’s more current use: a thing that is hoped or wished for but that is in fact illusory or impossible to achieve.

AFRL Wants Your Opinion. The Air Force Research Laboratory seeks feedback from the PNT community on the Chimera enhancement for the L1C signal. The specification is here. And, you can download a comment form

<p>The post AFRL tests Chimera to battle spoofers and hackers first appeared on GPS World.</p>

The letter sprang from a unanimous vote five days earlier to oppose allowing Ligado Networks to use spectrum neighboring the GPS band for terrestrial communications.

Ligado possesses licenses to broadcast on two satellite bands located adjacent to the GPS frequencies. The company has been seeking permission from the Federal Communications Commission (FCC) to repurpose these licenses from satellite-based use to ground-based use from powerful tower transmitters.

Ligado said in May it would lower the power in its proposal for the 1526–1536 MHz band to 9.98 dBW to avoid interference with certified aviation receivers. However, the PNT Advisory Board reiterated its opposition, saying that even if the transmissions’ power was lowered to just under 10 watts, it “will create totally unacceptable interference for a great number of GPS users in the United States.”

From the Letter: “This risk is far too great, and far too many questions remain, for Ligado’s proposal to be approved. While there are many broadband alternatives (Ligado would be a very small percentage of this national asset), there is only one GPS. Any impairment to current and future uses is clearly contrary to the national interest. Therefore, implementation of their recently proposed ~10-watt operating scheme will create totally unacceptable interference for a great number of GPS users in the United States. In fact, despite power limits in their current amended application, it is probable they could still be allowed to increase this power over time. This would be even more destructive to GPS users.

“We believe avoiding degradation over at least 90 percent of the region near Ligado transmitters is the absolute minimum protection for GPS receivers in each class. This would be a hypothetical 90 percent Protection Evaluation. This is not an endorsement of this level since, of course, all users would prefer 100 percent protection. The Department of Transportation (DOT) Adjacent Band Compatibility (ABC) study is the only validated test to verify degradation at various received power levels.

“Those results inform that to insure degradation not exceed 10 percent of the Region (90 percent Protection) for High Performance receivers, either:

Ligado maximum power can be no more than .0036 watts at the 400-meter spacing they had earlier planned. Tolerable power would be 3/10ths of 1 percent of their proposed ~10 watts. Or

the closest spacing of Ligado transmitters is 20,000 meters (over 12 miles) for their proposed ~10 watt power level (see Figure 1).”

Figure 1. The PNTAB strongly believes that 90 percent is the minimum Area Protection Criterion (maximum 10 percent degradation). (Chart: PNT Advisory Board)

DARPA wants photonic integrated circuits

High-energy photons emission (abstract illustration). (Photo: GiroScience/Shutterstock.com)

The U.S. Defense Advanced Research Projects Agency (DARPA) Microsystems Technology Office is soliciting research proposals for the development of a new class of atom-based systems using integrated photonics and trapped atoms to enable high-performance, robust, portable clocks and gyroscopes.

The military researchers are asking industry to develop relatively simple portable photonic integrated circuits (PICs) for high-performance position, navigation and timing (PNT) devices as an alternative to GPS for when satnav signals are not available.

A PIC or integrated optical circuit, similar to an electronic integrated circuit, integrates multiple photonic (having to do with light) functions, providing capabilities for information signals imposed on optical wavelengths, typically in the visible spectrum or near-infrared, 850–1650 nanometers.

A-PhI Program

The Atomic-Photonic Integration (A-PhI) program seeks to develop trapped-atom based, high-performance PNT devices, reducing the complexity of these atomic systems by using PICs. According to the DARPA document, the PICs will replace the optical assembly behind devices such as sensitive and accurate angle sensors and clocks, while still enabling the necessary trapping, cooling, manipulation and interrogation of atoms.

A-PhI aims to demonstrate that compact PICs can replace the optical bench of conventional free-space optics for high-performance trapped-atom gyroscopes and trapped-atom clocks without degrading the performance of the underlying physics package.

Physics

Atomic systems using trapped atoms have the potential to be made portable while maintaining their accuracy due to the atomic trap’s small size and the inherent isolation a trap offers an atomic system from the environment, especially from acceleration.

Currently, these systems are bulky, heavy, and not notably portable, because of the complexity of the optical systems used to create the trap.
In the past, efforts to miniaturize the hundreds to thousands of optical components in such benchtop systems have relied on removing optical elements, miniaturizing the remaining elements, and tightly integrating them in a small package.

The products deliver degraded performance with the need to maintain very tight optical alignment, causing both poor environmental robustness and poor tolerance to design errors. Effective miniaturized atomic systems cannot be achieved at a reasonable cost with this approach.

Recent developments in PIC research suggest that on-chip optical frequency combs based on microresonators, optical frequency synthesis, novel on-/off-chip coupling, wavelength demultiplexers, and on-chip phased arrays for dynamic manipulation of light fields can replace optical systems with readily manufacturable, low-cost chips without the alignment sensitivity of conventional free-space optics.

Gyroscopes

A-PhI also seeks to develop proof-of-concept trapped atom gyroscopes, a matter-wave analog of the interferometric fiberoptic gyroscope. Such a miniaturization effort could generate an order of magnitude improvement in angular sensitivity and dynamic range over current free-space products.

A-PhI hopes to develop portable, high-performance, navigation and timing systems: the miniaturization of the optics of atomic systems without a decrease in performance. Subsequent work, the RFP asserts, will be required to incorporate the necessary compact and robust lasers and electronics to achieve a fully functioning, high-performance, portable PNT system.

<p>The post PNT Board opposes Ligado ‘lite’ proposal, DARPA seeks photonics first appeared on GPS World.</p>